Pressure sensitive adhesives and adhesive-coated sheet materials are well-known in the art as having the ability to adhere to adhesive-receptive surfaces on mere contact. Such coated sheets in the form of labels and tapes, for example, can be adhered to various substrates under very light "finger pressure" and can later be removed therefrom by stripping the sheet from the surface to which it is attached. Such sheets cannot, however, be slid around on the surface of the substrate with the adhesive coating in contact with the substrate because of the inherent "quick-grab" properties (i.e., high initial adhesion strength) of conventional pressure sensitive adhesive (PSA) compositions.
The precise positioning of PSA-coated sheet materials is extremely difficult if not impossible to achieve, even immediately after their initial application to the substrate. This problem is particularly apparent in the case of decorative sheets where the matching of patterns on adjacent sheets is required. Folds, wrinkles, and bubbles may also appear and are often difficult to remove because of the high initial adhesion of the PSA. In addition, the inherently narcissistic character (i.e., property of "self-stickiness") of PSAs can cause problems when the sheet is folded on itself to cause areas of the PSA coating to be accidentally brought together. Attempts to pull the adhered areas apart often result in discontinuities due to removal of areas of the PSA coating. Torn, stretched, or wrinkled areas of the facing sheet can also result.
It is therefore often desirable to reduce the initial affinity of a PSA for a substrate while still maintaining sufficient ultimate adhesion (after pressure bonding). The prior art has attempted to achieve this result in various ways, mainly by use of different types of "physical spacers" which have been mixed with or coated on the surface of the PSA. Such spacers act to hold the adhesive slightly away from the substrate surface until the desired placement has been achieved. For example, U.S. Pat. Nos. 3,314,838, 3,331,729, and 3,413,168, all assigned to the present assignee, disclose the use of hollow, spheroidal particles or microballoons composed of glass, urea- or phenol-formaldehyde resins, etc., to decrease initial adhesion and thereby provide positionability. The microballoons are described as being crushable or collapsible under applied hand pressure, thereafter enabling the PSA to contact and adhere to the surface of the substrate. Fragments of the collapsed or crushed microballoons may, however, remain on the bonding surface of the adhesive rather than completely dispersed into the adhesive layer, and this may, at least initially, adversely affect the adhesion strength.
In U.S. Pat. Nos. 4,054,697 (Imperial Chemical Industries Limited) and 4,556,595 (Nippon Carbide Kogyo Kabushiki Kaisha), positionability is obtained via the use of non-adhesive, solid particles as "physical spacers". The former patent utilizes resilient particles of natural or synthetic rubber (or blends thereof) or a foamed organic polymeric material such as polyurethane. Once positioned, the PSA-coated sheet material of this invention is again adhered to the surface of a substrate via the application of hand pressure. This serves to deform (rather than to crush or collapse) the particles on the surface of the PSA to such an extent as to bring the adhesive and the substrate into fuller contact. Use of the resilient, solid particles eliminates potential problems due to particle fragments on the adhesive surface, but the potential for "particle recovery" puts constraints on the PSA. Only those PSAs which prevent the deformed particles from recovering to their undeformed dimensions are acceptable, as such recovery will result in reduced adhesion to the substrate. The former patent ('697) states that, in practice, it may be desirable for the PSA to be curable after pressure bonding to the substrate surface, in order to overcome any tendency of the deformed particles to recover. The latter patent ('595) describes the use of non-adhesive, solid particles (of either inorganic or organic materials) having an average diameter smaller than the thickness of the adhesive layer, so that pressure bonding acts to force the particles to move away from the PSA surface and become dispersed or embedded in the bulk PSA. As with any use of a "physical spacing means" to provide positionability, an additional manufacturing step is required in order to distribute the particles or spacers of choice.
Silicone has also been used as a spacing material to reduce the initial adhesion of PSA coatings and thereby produce positionable products. For example, U.S. Pat. No. 3,554,835 (Morgan Adhesives Company) discloses a method of making and using a "slidable" PSA-coated laminate which relies upon "dots" of silicone or other conventional release material as non-adhesive spacers to enable initial positioning of the laminate on a substrate. Similarly, United Kingdom Pat. No. 1,541,311 (United Merchants and Manufacturers, Inc.) describes a PSA-coated laminate where positionability is provided by a uniform and discontinuous surface coating in the form of small beads or droplets of a non-tacky polysiloxane or polysiloxane-polyoxyalkylene block copolymer. Both teachings require an additional manufacturing step or steps to apply the silicone spacing material and to then effect solvent removal, drying, or curing.
In U.S. Pat. No. 4,151,319 (United Merchants and Manufacturers, Inc.) a method for making a positionable PSA-coated laminate is disclosed whereby polysiloxane or polysiloxane-polyoxyalkylene block copolymer is intimately mixed with the PSA itself rather than forming a coating on the PSA surface. "Pre-mixing" of a siloxane polymer or copolymer in a suitable solvent prior to incorporation into the adhesive is recommended. Again, the required mixing or dispersing and the recommended pre-mixing contribute additional process steps. The siloxane additives of this teaching are said to be in the form of small beads or droplets dispersed throughout the adhesive mass, and thus some of the droplets are positioned at the adhesive/release sheet interface of the laminate (i.e., at the PSA surface) and function to allow initial positioning on a substrate. None of the aforementioned patents utilizes chemically incorporated siloxane polymers or copolymers, and, therefore, uncontrolled loss of the non-adhesive polymeric siloxane materials to the release sheet could occur, especially upon storage and aging prior to use. The use of either block copolymers or high molecular weight (above 25,000) polysiloxanes is suggested in United Kingdom Pat. No. 1,541,311 and U.S. Pat. No. 4,151,319 to minimize such loss, but the resultant laminates still may not offer predictable levels of positionability even with minor losses.
U.S. Pat. No. 4,346,189 (Morgan Adhesives Company) describes the use of polysiloxane additives (of up to about 10,000 molecular weight) in a different type of application. The silicones are mixed with tackified, synthetic rubber-based PSA compositions to reduce edge ooze or flow upon cutting of sheets coated with such compositions. Either non-reactive or reactive polysiloxanes can be utilized and are said to appear to adsorb or graft onto other ingredients of the adhesives or take other unknown actions so as to produce the desired effect. It is stated, however, that the silicones can be added even at relatively high concentrations (6 to 10% solids) without adversely affecting the adhesive characteristics of the material.
Graft copolymers, some containing silicone, are being increasingly used for general modification of surface properties, as is described in a series of papers by Y. Yamashita et al., [Polymer Bulletin 7, 289 (1982); Polymer Bulletin 10, 368 (1983); Makromol. Chem. 185, 9 (1984); Macromolecules 18, 580 (1985)]. Such use is also reflected in some recent Japanese art, such as Japanese Patent Application No. 57-179246, published Nov. 4, 1982, which concerns the use of graft copolymers as additives to impart long-lasting hydrophobicity (or hydrophilicity) to surfaces. In Japanese Patent Applications Nos. 58-167606, published Oct. 3, 1983, and 58-154766, published Sept. 14, 1983, a method of preparation of silicone graft copolymers and the use of these copolymers in coating compositions such as paint films are described. Here, the copolymers are said to provide long-lasting water- and oil-repellency, stain resistance, and reduced frictional properties. Japanese Patent Application No. 59-78236, published May 7, 1984, discloses a method of preparing monofunctional polymeric silicone monomers, i.e., macromonomers, for use in the preparation of graft copolymers as surface-treatment agents. The use of such silicone macromonomer-grafted copolymers in coating compositions, again to impart lasting water- and oil-repellency, stain resistance, and low friction characteristics, is described in Japanese Patent Application No. 59- 126478, published July 21, 1984. Pending U.S. patent application Ser. No. 757,278, filed on July 19, 1985, assigned to the present consignee, discloses the use of tack-free polysiloxane-grafted copolymers (and blends thereof with other polymeric materials) as release coating compositions for PSA-coated sheet materials and the back side of PSA-coated tapes.
We are unaware of any art, however, which teaches the use of chemically tailored, tacky or tackified, polysiloxane-grafted copolymers as temporarily positionable pressure sensitive adhesives for controlled initial adhesion applications.